(a) Field of the Invention
An exemplary embodiment of the present invention relates to a fuel cell stack of a fuel cell system. More particularly, the present invention relates to a separator having a microporous structure and a fuel cell including the separator.
(b) Description of the Related Art
As known in the art, a fuel cell is composed of unit cells that generate electric energy from an electrochemical reaction of hydrogen and oxygen. Such a fuel cell may be structured by arranging separators at both sides with a membrane-electrode assembly (MEA) therebetween. Reaction channels for supplying fuel and a reaction gas (e.g., air) to the membrane-electrode assembly and a cooling channel for passing cooling water are formed in the separators. A gas diffusion layer for diffusing the reaction gas is formed on both sides of the membrane-electrode assembly.
To maximize the performance of the fuel cell, the surface pressure of the gas diffusion layers and the membrane-electrode assembly require uniformity achieved by narrowing the gap between the reaction channels of the separators and uniform permeability is achieved throughout the reaction surfaces of the gas diffusion layers. However, there is a limit in reducing the gap between the reaction channels of the separators to prevent various defects caused in the process of forming of the separators and the following factors decreasing the performance of the fuel cell are generated due to this practical problem.
First, when the gap between the reaction channels is substantial, stress concentrates on the contact surface between the separator and the gas diffusion layer. Accordingly, the porous structure of the gas diffusion layer may break thus causing the permeability for the reaction gas to deteriorate, and the ability of diffusing the reaction gas and the ability of discharging product water may decrease. Further, since stress is minimal in the surface where the reaction channels are formed, the gas diffusion layer protrudes from the channel portions of the separator, such that fluidity of fluid may be deteriorated.
Second, as the structure of the gas diffusion layer is broken due to the concentration on the contact surface of the separator, carbon fibers may permeate to an electrode layer of the membrane-electrode assembly through the broken areas, such that the electrode layer may be damaged.
Third, in the channel portions with the gas diffusion layer exposed, the reaction gas is supplied sufficiently and an active chemical reaction may occur, but contact resistance may increase due to lack of surface pressure between the gas diffusion layer and the membrane-electrode assembly, causing potential difficulty in the movement of electrons produced by the reaction.
To improve the above problems, a formed-porous member having a three dimensional (3D) porous structure by forming channels and a porous structure having micro-holes in a substantially thin metal plate has been used in the related art. Further, a method of inserting a microporous structure to uniformly distribute surface pressure and improve the abilities of diffusing a reaction gas and discharging product water, instead of a separator with reaction channels, has been used. Since a microporous structure such as metal foam and a wire mesh has a substantially high aperture ratio and distributes surface pressure, the gas diffusion layer may be compressed uniformly.
In those materials, the metal foam has a substantial number of bubbles connected with each other within the metallic material, and thus may pass fluid and have a substantially high ratio of surface area per volume and strength, and accordingly, it is suitable for the material of separators in fuel cells. However, in the related art, the most important defect of the metal foam is that it may be impossible to control the flow of a reaction gas and product water due to random connection of the internal bubbles and thus it may be difficult to efficiently use the entire reaction surface.
Further, since a microporous structure is used for separators in the related art, the pressure difference in the separators is substantially increased and thus parasitic power of a fuel cell system increases, a fuel cell increases in volume, and micropores may be clogged, with the fuel cell oversaturated with water (condensate water), such that operation safety of the fuel cell decreases.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.